As new emission laws require the reduction of tail pipe hydrocarbon emissions, engine cold start is a period when hydrocarbon emissions must be minimized. To reduce automotive vehicle emissions, substantial effort is being made to warm up catalytic treatment devices more quickly and to control engine air/fuel ratio close to stoichiometry following a vehicle cold start. High driveability index (low volatility fuel) causes the open loop air/fuel ratio during engine cold start to become lean and contribute to unstable combustion. In turn, the open loop air/fuel ratio is made richer, which consequently increases emissions when a lower driveability index fuel (higher volatility) is used. The driveability index is a value for measuring the expected performance during a vehicle cold start or drive-away. Better performance is expected from fuels with a lower driveability index value.
To meet certain emissions reduction goals, the exhaust stream from an engine to the catalytic treatment device must be substantially at stoichiometry or lean within seconds following a cold start. Providing a stoichiometric mixture of fuel and air within the combustion chamber of the engine requires good fuel injection control and tightly controlled fuel characteristics. Fuel injection control precision is improving. However, mid-range volatility of commercial fuel—a preferred fuel characteristic for exhaust stream air/fuel ratio control—can be highly variable depending on such factors as the geographical region, the season, and the feedstock. Accordingly, to ensure acceptable cold start driveability, the calibration used in engine air/fuel ratio control should be biased fuel rich of stoichiometry to anticipate a worst-case driveability index (volatility) fuel.
Automotive vehicles with an internal combustion engine have an exhaust system including a pathway for exhaust gas to move away from the engine. Depending on the desired operating state, internal combustion engines can be operated with air/fuel ratios in which (1) the fuel constituent is present in a stoichiometric surplus (rich range), (2) the oxygen of the air constituent is stoichiometrically predominant (lean range), and (3) the fuel and air constituents satisfy stoichiometric requirements. The composition of the air/fuel mixture determines the composition of the exhaust gas. In the rich range, considerable quantities of nonburned or partially burned fuel are found, while the oxygen has been substantially consumed and has nearly disappeared. In the lean range, the ratios are reversed, and in a stoichiometric composition of the air/fuel mixture, both fuel and oxygen are minimized.
It is well known that the oxygen concentration in the exhaust gas of an engine has a direct relationship to the air/fuel ratio of the fuel mixture supplied to the engine. As a result, gas sensors, namely oxygen sensors, are used in automotive internal combustion control systems to provide accurate oxygen concentration measurements of automobile exhaust gases for determination of optimum combustion conditions, maximization of fuel economy, and management of exhaust emissions.
A switch type oxygen sensor, generally, comprises an ionically conductive solid electrolyte material, a sensing electrode, which is exposed to the exhaust gas, and a reference electrode, which is exposed to the reference gas. It operates in potentiometric mode, where oxygen partial pressure differences between the exhaust gas and reference gas on opposing faces of the electrochemical cell develop an electromotive force, which can be described by the Nernst equation:   E  =            (              RT                  4          ⁢                                           ⁢          F                    )        ⁢          ln      ⁡              (                              P                          O              2                        ref                                P                          O              2                                      )                            where: E=electromotive force        R=universal gas constant        F=Faraday constant        T=absolute temperature of the gas        PO2ref=oxygen partial pressure of the reference gas        PO2=oxygen partial pressure of the exhaust gas        
The large oxygen partial pressure difference between rich and lean exhaust gas conditions creates a step-like difference in cell output at the stoichiometric point; the switch-like behavior of the sensor enables engine combustion control about stoichiometry. Stoichiometric exhaust gas, which contains unburned hydrocarbons, carbon monoxide, and oxides of nitrogen, can be converted very efficiently to water, carbon dioxide, and nitrogen by automotive three-way catalysts in automotive catalytic converters. In addition to their value for emissions control, the sensors also provide improved fuel economy and drivability.